1. Field of the Invention
The disclosed invention relates to lighting devices including light-emitting elements, and display devices and electronic devices including lighting devices.
2. Description of the Related Art
In recent years, display devices and lighting devices that include light-emitting elements typified by organic EL (electroluminescence) elements emitting light at a luminance of 100 to 1000 cd/m2 have been developed. Broader applications have been expected by taking advantages of high image quality due to self emission, wide viewing angle, small thickness, lightweight, and the like. In particular, applications of organic EL elements to lighting equipment have been actively progressed. However, the life of organic EL elements and the emission efficiency of the organic EL elements decrease with a change over time. Thus, more improvement in luminance and more suppression of deterioration have been needed.
The resistance of light-emitting elements increases and the luminance of the light-emitting elements fluctuates as time passes. In Reference 1, fluctuation in luminance is suppressed by constant-current drive of a light-emitting element. Concerning deterioration of light-emitting elements, stable drive at high temperature for a long time tends to be emphasized. Even when ambient temperature is room temperature, the temperature of light-emitting elements is raised by generation of heat due to internal resistance at the time of being driven; thus, the light-emitting elements need endurance at high temperature.
As a circuit for converting input voltage into desired output voltage, a linear regulator and a switching regulator are known. The switching regulator obtains voltage at a desire level in such a manner that voltage with a pulse waveform is formed from input voltage by a switching element and the voltage is smoothed or held in a coil, a capacitor, or the like. In such a regulator, a slight increase in conversion efficiency is directly linked to a reduction in power consumption. Thus, development of a technique for performing efficient control is needed.